The Law of diminishing returns is a key one in economics. It is used to explain many of the ways the economy works and changes. It is a relatively simple idea; spending and investing more and more in a product where one of the factors of production remains the same means the enterprise will eventually run out of steam. The returns will begin to diminish in the long run. If more fertilizer and better machinery are used on an acre of farmland, the yield will increase for a while but then begin to slow and become flat. A farmer can only get so much out of the land, and the more the farmer works, the harder it gets. The economic reason for diminishing returns of capital is as follows: When the capital stock is low, there are many workers for each machine, and the benefits of increasing capital further are great; but when the capital stock is high, workers already have plenty of capital to work with, and little benefit is to be gained from expanding capital further. For example, in a secretarial pool in which there are many more secretaries than computer terminals, each terminal is constantly being utilized and secretaries must waste time waiting for a free terminal. In this situation, the benefit in terms of increased output of adding extra terminals is high. However, if there are already as many terminals as secretaries, so that terminals are often idle and there is no waiting for a terminal to become available, little additional output can be obtained by adding yet another terminal.
Another application for this law is in Athletics, for runners, their investment is the time and energy put into training and the yield is hopefully improved fitness. Early in their running careers or early in the training program a couple of weeks of regular training would be rewarded with a considerable increase in fitness. Having achieved a very fit state though, two weeks of regular training will achieve a barely perceptible increase in fitness.
But in todays world, this famous law seems to have been turned on its head. In Japan, for example, huge amounts of investment have resulted in large increases in the economy and large increases in capital goods per worker. But the rate of productivity growth did not decline the way one would have expected on the basis of diminishing returns. Japan got ahead and stayed ahead.